feat(mobile): cache latest ios widget entry for fallback (#19824)

* cache the last image an ios widget fetched and use if a fetch fails in a future timeline build

* code review fixes

* downgrade pbx for flutter

* use cache in snapshots

mobile/ios/WidgetExtension/ImageEntry.swift

@@ -0,0 +1,161 @@
+import SwiftUI
+import WidgetKit
+
+typealias EntryMetadata = ImageEntry.Metadata
+
+struct ImageEntry: TimelineEntry {
+  let date: Date
+  var image: UIImage?
+  var metadata: Metadata = Metadata()
+
+  struct Metadata: Codable {
+    var subtitle: String? = nil
+    var error: WidgetError? = nil
+    var deepLink: URL? = nil
+  }
+
+  // Resizes the stored image to a maximum width of 450 pixels
+  mutating func resize() {
+    if image == nil || image!.size.height < 450 || image!.size.width < 450 {
+      return
+    }
+
+    image = image?.resized(toWidth: 450)
+
+    if image == nil {
+      metadata.error = .unableToResize
+    }
+  }
+
+  static func build(
+    api: ImmichAPI,
+    asset: Asset,
+    dateOffset: Int,
+    subtitle: String? = nil
+  )
+    async throws -> Self
+  {
+    let entryDate = Calendar.current.date(
+      byAdding: .minute,
+      value: dateOffset * 20,
+      to: Date.now
+    )!
+    let image = try await api.fetchImage(asset: asset)
+
+    return Self(
+      date: entryDate,
+      image: image,
+      metadata: EntryMetadata(
+        subtitle: subtitle,
+        deepLink: asset.deepLink
+      )
+    )
+  }
+
+  func cache(for key: String) throws {
+    if let containerURL = FileManager.default.containerURL(
+      forSecurityApplicationGroupIdentifier: IMMICH_SHARE_GROUP
+    ) {
+      let imageURL = containerURL.appendingPathComponent("\(key)_image.png")
+      let metadataURL = containerURL.appendingPathComponent(
+        "\(key)_metadata.json"
+      )
+
+      // build metadata JSON
+      let entryMetadata = try JSONEncoder().encode(self.metadata)
+
+      // write to disk
+      try self.image?.pngData()?.write(to: imageURL, options: .atomic)
+      try entryMetadata.write(to: metadataURL, options: .atomic)
+    }
+  }
+
+  static func loadCached(for key: String, at date: Date = Date.now)
+    -> ImageEntry?
+  {
+    if let containerURL = FileManager.default.containerURL(
+      forSecurityApplicationGroupIdentifier: IMMICH_SHARE_GROUP
+    ) {
+      let imageURL = containerURL.appendingPathComponent("\(key)_image.png")
+      let metadataURL = containerURL.appendingPathComponent(
+        "\(key)_metadata.json"
+      )
+
+      guard let imageData = try? Data(contentsOf: imageURL),
+        let metadataJSON = try? Data(contentsOf: metadataURL),
+        let decodedMetadata = try? JSONDecoder().decode(Metadata.self, from: metadataJSON)
+      else {
+        return nil
+      }
+
+      return ImageEntry(
+        date: date,
+        image: UIImage(data: imageData),
+        metadata: decodedMetadata
+      )
+    }
+
+    return nil
+  }
+
+  static func handleCacheFallback(
+    for key: String,
+    error: WidgetError = .fetchFailed
+  ) -> Timeline<ImageEntry> {
+    var timelineEntry = ImageEntry(
+      date: Date.now,
+      image: nil,
+      metadata: EntryMetadata(error: error)
+    )
+
+    // skip cache if album not found or no login
+    // we want to show these errors to the user since without intervention,
+    // it will never succeed
+    if error != .noLogin && error != .albumNotFound {
+      if let cachedEntry = ImageEntry.loadCached(for: key) {
+        timelineEntry = cachedEntry
+      }
+    }
+
+    return Timeline(entries: [timelineEntry], policy: .atEnd)
+  }
+}
+
+func generateRandomEntries(
+  api: ImmichAPI,
+  now: Date,
+  count: Int,
+  albumId: String? = nil,
+  subtitle: String? = nil
+)
+  async throws -> [ImageEntry]
+{
+
+  var entries: [ImageEntry] = []
+  let albumIds = albumId != nil ? [albumId!] : []
+
+  let randomAssets = try await api.fetchSearchResults(
+    with: SearchFilters(size: count, albumIds: albumIds)
+  )
+
+  await withTaskGroup(of: ImageEntry?.self) { group in
+    for (dateOffset, asset) in randomAssets.enumerated() {
+      group.addTask {
+        return try? await ImageEntry.build(
+          api: api,
+          asset: asset,
+          dateOffset: dateOffset,
+          subtitle: subtitle
+        )
+      }
+    }
+
+    for await result in group {
+      if let entry = result {
+        entries.append(entry)
+      }
+    }
+  }
+
+  return entries
+}